Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process via cumene. This is a three-step process in which the first step involves alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for acetone. In addition, due to a developing shortage, the cost of propylene is likely to increase. Thus, a process that uses higher alkenes instead of propylene and coproduces higher ketones, rather than acetone, may be an attractive alternative route to the production of phenols.
One such process involves the hydroalkylation of benzene to produce cyclohexylbenzene, followed by the oxidation of the cyclohexylbenzene (analogous to cumene oxidation) to cyclohexylbenzene hydroperoxide, which is then cleaved to produce phenol and cyclohexanone in substantially equimolar amounts. Such a process is described in, for example, U.S. Pat. No. 6,037,513.
However, although cyclohexanone is a valuable product with a growing market, there is currently no large worldwide merchant market for cyclohexanone and so most cyclohexanone is made as an intermediate and consumed on the spot. To obviate this problem it has been proposed to integrate the cyclohexylbenzene oxidation process with a dehydrogenation step whereby at least part of the cyclohexanone is converted to additional phenol (see co-pending International Patent Application Serial No. PCT/US2009/037223). To date, however, only limited research has been conducted into optimizing the process of dehydrogenating cyclohexanone to phenol.
For example, U.S. Pat. No. 3,534,110 discloses a process for the catalytic dehydrogenation of cyclohexanone and/or cyclohexanol to phenol over a catalyst comprising platinum and preferably iridium on a silica support, which also contains 0.5 to 3 wt % of an alkali or alkaline earth metal compound.
U.S. Pat. No. 4,933,507 discloses that phenol can be produced by dehydrogenating cyclohexenone through a vapor-phase reaction in the presence hydrogen using a solid-phase catalyst having platinum and alkali metal carried on a support. The catalyst support proposed in the '507 patent is silica, silica-alumina or alumina.
U.S. Pat. No. 7,285,685 discloses a process for the dehydrogenation of a saturated carbonyl compound, such as cyclohexanone, in the gas phase over a heterogeneous dehydrogenation catalyst comprising platinum and/or palladium and tin on an oxidic support, such as zirconium dioxide and/or silicon dioxide. In addition, the dehydrogenation catalyst can further comprise one or more elements of Groups I and/or II, preferably potassium and/or cesium, which are added to the catalyst as aqueous solutions of compounds which can be converted into the corresponding oxides by calcination.
One problem that has been encountered in the use of supported noble metal catalysts in the dehydrogenation of cyclohexanone is that the activity of the noble metal decreases fairly rapidly with time on stream, especially at economically attractive space velocities. There is therefore a need for a simple, cost effective and non-destructive process for periodically recovering the catalyst activity. However, although air calcination is frequently used to reactivate noble metal-containing catalysts, it has been found that using air calcination tends to be accompanied by migration and agglomeration of the noble metal on the support thereby resulting in reduction in metal surface area and therefore a reduction in catalyst activity.
According to the present invention, it has now been found that contacting the catalyst with hydrogen at a temperature of at least 300° C. is effective in restoring the activity of a supported noble metal catalyst used in the dehydrogenation of cyclohexanone. In addition, an analogous process, high temperature contact of the catalyst with a non-oxidizing atmosphere, such as hydrogen or nitrogen, can be used to protect the catalyst during temporary idling of the cyclohexanone dehydrogenation reactor required, for example, to allow maintenance on the dehydrogenation product separation train.